If you have many job setups and woulld like to retain all the device information, etc after the migration from Backup Exec 9.1 to Backup Exec 10.0 then you can go for an upgrade as it will keep all your settings intact.On the other hand, you will have to uninstall Backup Exec 9.1 which means that you will be recreating all the device configuration, job setups, etc if Backup Exec 10.0 is installed from scratch. We would recommend you to upgrade from 9.1 to Backup Exec 10.0
We hope this helps.